A tape library device having magnetic tapes as a plurality of recording media is widely used to back up a large amount of data. The advantage of the magnetic tape is its low cost and large volume capacity while the disadvantage of the magnetic tape is that a data access speed thereof is slower than a data access speed of a hard disk drive (HDD). A hierarchical storage system has been recently used. In the hierarchical storage system, a disk array device including a plurality of HDDs is used as a cache mechanism (primary storage), and an even larger capacity tape library device is used as a back-end (secondary) storage.
The hierarchical storage system performs a process described below. If a data write request is received from a host device, the corresponding data is written first onto the primary storage. When the data write process is complete, a write process completion is reported back to the host device. The primary storage then writes the written data onto the secondary storage. If a data read request is received from the host device, the corresponding data, if stored on the primary storage, is transferred to the host device. If no corresponding data is stored on the primary storage, the data is read from the secondary storage and then written onto the primary storage. When the data write process is completed, the data is transferred from the primary storage to the host device.
However, once the primary storage fails in such a system, no access at all is permitted, regardless of whether the access is for the write process or the read process. For example, a read request may now be received from the host device. Even if the effective data is present on the secondary storage, the host device has difficulty in retrieving the data with the primary storage malfunctioning.
In a related art technique, a hierarchical storage system may include a fault-tolerance primary storage having a currently operative storage section and a backup storage section. A logical volume of a faulty current storage section is restored onto the backup storage section. However, this technique needs the backup storage section, leading to an increase in the manufacturing cost of the primary storage.
Another related art technique is related to a computer system. In the event of a memory failure, a copy of a faulty page is output from a trace cache to a secondary storage to restore the faulty page on the computer system. In yet another related art technique, if no hot spare is available on a primary storage having a mirroring structure during the occurrence of a disk degradation, another disk pair is disengaged to use one of the pair members as a hot spare.
When the primary storage malfunctions in the hierarchical storage system, accessing from the host device for writing and reading becomes difficult, and the operation of the system is suspended.
The followings are reference documents.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2006-190039
[Patent Document 2] Japanese Laid-open Patent Publication No. 9-81464
[Patent Document 3] International Publication Pamphlet No. WO 2006/085357